1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer or copying apparatus, a control method therefor, a control program, and a storage medium and, more particularly, to a technique of controlling the print speed of an image forming apparatus having an intermediate transfer member.
2. Description of the Related Art
In printing on a transfer medium with a large heat capacity such as an OHP film, thick paper, or envelop, a full-color image forming apparatus decreases the transfer medium conveying speed in comparison with that in printing on normal plain paper, in order to avoid degradation of the fixing performance of a fixing unit. This operation prolongs the time during which a transfer medium passes through the fixing unit. The amount of heat applied from the fixing unit to the transfer medium can increase, and the fixing temperature can rise to a high level. Even when printing on a transfer medium with a large heat capacity, a stable fixing characteristic can be ensured without degrading the fixing performance of the fixing unit.
In a conventional one-drum type full-color image forming apparatus having an intermediate transfer member, a toner image is formed on the photosensitive member and is transferred onto the intermediate transfer member (to be referred to as primary transfer hereinafter). In primary transfer, the photosensitive member and intermediate transfer member rotate at a normal print speed. Then, the driving speed is switched to perform a subsequent process of transferring a toner image from the intermediate transfer member onto a transfer medium (to be referred to as secondary transfer hereinafter). However, the one-drum type image forming apparatus takes a long primary transfer time because toner images developed using one photosensitive member and a plurality of developing units are superposed and transferred onto the intermediate transfer member for each respective color.
These days, a four-drum type image forming apparatus has appeared, which can simultaneously form toner images in respective colors on the intermediate transfer member by using four photosensitive members and developing units. In the four-drum type image forming apparatus, the first toner image is formed on the first photosensitive member and primarily transferred from the photosensitive member onto the intermediate transfer member. Then, the second toner image is formed on the second photosensitive member and transferred over the first toner image on the intermediate transfer member. This operation is continuously executed for four photosensitive members (e.g., B, C, M, and Y photosensitive members). As a result, a color image is formed from the four superposed toner images on the intermediate transfer member. The color image formed on the intermediate transfer member is secondarily transferred from the intermediate transfer member to a transfer medium at the secondary transfer position (e.g., Japanese Patent Laid-Open No. 2004-020616).
When successively forming images based on a plurality of print jobs, the four-drum type image forming apparatus parallel-executes a primary transfer process for a subsequent print job and a secondary transfer process for a preceding print job. For this reason, the primary and secondary transfer processes are equal in speed. When the print speed is to be switched upon changing the print mode or the like, image forming processes such as the primary and secondary transfer processes temporarily end. Then, the primary and secondary transfer units switch to a print speed corresponding to the selected mode, and the image forming processes start again.
The above-mentioned four-drum type image forming apparatus with an intermediate transfer member requires a longer length from the position of the primary transfer unit for performing the primary transfer process to that of the secondary transfer unit for performing the secondary transfer process. To switch the print speed, an image forming process in progress must be temporarily stopped. This prolongs the time until the next primary transfer process is executed.
If the image forming process is to restart from the primary transfer process of an image after switching the speeds of the primary and secondary transfer units upon switching the print speed, like the prior art, the throughput greatly decreases upon speed switching.